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What is the use of the normal force or flow rate when defining a periodic boundary condition in POLYFLOW?

When the periodic boundary condition involves a rotation only, there is formally no pressure difference between the source and target sides. The main reason is that one comes back to the source side after 360°, and one must of course recover the same pressure level. For a similar reason, the flow rate does not need to be specified, as it will result from the other boundary conditions. However, there are some specific modeling issues where one may want to impose a non-vanishing pressure gradient.
When the periodic boundary condition involves a translation, a pressure drop usually exists between the source and target, and that one should be specified. Alternatively, a flow rate can be assigned. One cannot simultaneously impose the pressure drop and the flow rate.